Federal safety experts worked feverishly yesterday to protect thousands of rescuers as they stepped over smoking debris – and into a potential land mine of lung-choking asbestos and cornea-scratching silica.

The asbestos level in the billowing smoke enveloping workers digging for survivors was four times the safe level, said one cop at the scene of the World Trade Center.

“There is concern regarding rescue workers and asbestos and silica (the sandy residue from demolished cement and sheet rock),” acknowledged Bill Wright, spokesman for the U.S. Occupational Safety and Health Organization.

“Normally, employees dealing with asbestos require eight hours of training. Obviously we can’t do this here.”

Wright said OSHA was sending teams of inspectors to try to guide workers through the protocol of handling the material.

The federal Emergency Management Agency and Environmental Protection Agency also were testing for asbestos and dioxin.

So far, three air samples showed “minimal or low” levels of cancer-causing airborne asbestos, said EPA chief Christie Todd Whitman. But a fourth sampling sounded the alarm by detecting significant levels of the material.

Mike Borello, director of an asbestos-testing firm in Midtown, said debris sampling that his firm did for several worried businesses near the site showed asbestos levels were less than 1 percent.

“The thing that surprised us was that we found fiberglass,” Borello said, adding that fiberglass may be irritating workers’ eyes and throats.